The present invention relates to methods and apparatus for measuring the QT interval on an electrocardiogram.
An electrocardiogram (ECG) measures the instantaneous voltage potential difference of myocardial electrical activity in a number of lead vectors. “Leads” (or signals) are obtained from a subject using electrodes placed on their skin, placed on standardised locations around the body. Each electrode is connected to a signal processing apparatus via a respective wire (or “lead”). The term “lead” commonly refers to either a physical wire to an electrode or to an ECG signal itself. Commonly 12 different leads are used although it is also known to collapse all the information to within three orthogonal X, Y and Z leads or three quasi-orthogonal leads consisting of known ECG leads I, aVf and V2.
The durations of certain types of waves within the ECG give very important information. One important interval is the QT interval which is approximately 400 ms in duration. Variation in the QT interval is a known indicator of cardiac dysfunction. For example, an average increase in the QT interval of as little as 5-10 ms during pharmaceutical compound safety testing indicates the potential for the drug to induce a fatal cardiac rhythm disturbance if the compound were to be more widely prescribed. Therefore accurate measurement of the QT interval in the standard 12 lead ECG is of paramount importance when assessing the safety of a new drug.
Although the onset of the QT interval can be accurately measured by existing methods, there are difficulties measuring the accurate timing of the end of the “T wave” of the ECG due to the low frequency nature of the waveform, superaddition of an ECG “U wave”, baseline drift of the signal and other superadded high frequency noise.
Automated methods to measure the QT interval with good reproducibility in a given ECG vector lead exist. However, they are not considered any more accurate than expert manual measurements hence they can give reproducibly wrong measurements. Even if the current automated methods to measure the QT interval were accurate, errors in measuring the real longest QT interval for the median signal of a 12 lead ECG vector arise because, in addition to the three quasi-orthogonal leads I, aVf and V2 which contain the longest QT interval information, there are nine further lead vectors containing noise and U waves which would contaminate the information from the orthogonal leads, causing measurement error.